The present invention relates to photonic crystal materials (also known as photonic bandgap materials) and a method of preparation thereof. In general a photonic crystal is a composite dielectric medium having a structure which varies periodically on a length scale comparable to the wavelength of electromagnetic radiation. In an ideal photonic crystal multiple scattering and interference of electromagnetic waves propagating through the medium results in forbidden frequency bands for a given direction of propagation within which no propagating electromagnetic modes exist. Within a forbidden band the material is highly reflective and inside the medium emission of radiation having frequencies in the forbidden band is suppressed. A photonic crystal structure may be employed to modify the interaction of a material with electromagnetic radiation, including for the purpose of controlling the appearance of the surface of the material, and in the construction of optical and optoelectronic devices (not restricted to visible optical frequencies).
In U.S. Pat. No. 5,385,114 a method of preparing a photonic crystal is described in which the pores of a reticulated mesh are impregnated with a suitable liquid dielectric material which is then solidified. In order to introduce the dielectric material, the material of the mesh must have a much higher melting point than the dielectric material and so, for example, the material of the mesh is a metal. Thereafter, the mesh is dissolved using a suitable liquid chemical reactant to leave a porous dielectric material. The pores of the dielectric material have a different refractive index to the material itself, so a periodic structure made in this way would enable the material to function as a photonic crystal. After the reticulated mesh has been removed, the pores in the dielectric material may be filled with a separate material that has a refractive index different to the refractive index of the dielectric material. In this document the method of pore filling is demonstrated using a random rather than a periodic metallic mesh but it is envisaged that a periodic metal mesh could be formed by freezing electro-hydrodynamically generated metal droplets, by weaving a mesh of wires, by assembling small pieces or, by inference from the preamble, by drilling or reactive ion etching a slab of metal through a mask.
In order for the photonic crystal to be useful, the periodicity of the dielectric material must be of size comparable to the electromagnetic wavelength of interest. On a scale comparable to visible optical or near-infrared wavelengths only the etching procedures are practicable. However, fabrication of masks suitable for use in such drilling or ion etching operations is extremely difficult and costly. Such techniques currently cannot provide the necessary resolution or drilling depth to produce photonic crystals for use at visible optical wavelengths.
The present invention seeks to provide a novel photonic crystal material and a method of preparing the same, having a three-dimensional periodicity with a length scale comparable to infra-red, visible, optical or shorter electromagnetic wavelengths, which overcomes the disadvantages of the known procedures described above. Reference to three dimensional periodicity is intended as reference to periodic variation of a characteristic in all three dimensions of the material. Reference to periodicity or periodic variations should be taken to include the cases in which the variation is substantially periodic and cases in which two or more periodic patterns, with unit cells which may not be commensurable, are superimposed. Reference to the length scale of a periodic variation is intended to refer to a characteristic dimension of a Wigner-Seitz primitive unit cell.
The present invention provides a method of forming a photonic crystal material comprising irradiating a sample of photosensitive material with electromagnetic radiation such that interference between radiation propagating in different directions within the sample gives rise to a three dimensional periodic variation of the intensity of irradiation within the sample whereby the periodic variation in intensity produces a corresponding periodic variation in the refractive index of the photosensitive material.
Ideally, the irradiated sample of photosensitive material is developed to remove either more irradiated or less irradiated regions of the sample.
To maintain the three dimensional periodicity within the sample, the intensity interference pattern is not substantially perturbed by photoinduced changes in the refractive index of the sample material. The sample of photosensitive material may be subjected to multiple exposures each producing respective interference patterns within the sample.
The sample of photosensitive material may be irradiated with a coherent or partially coherent source of electromagnetic radiation. Material may be introduced into the voids in the composite material or the composite material may be used as a template for the production of other composite materials having periodic variations in refractive index. By both of these techniques the optical properties of the photonic crystal material may be altered and the frequency ranges of the forbidden photonic band gaps adjusted. In particular, by selection of a material having an appropriate refractive index, the overlap between forbidden frequency bands corresponding to different directions of propagation of radiation may be increased to create or to widen a complete photonic band gap, i.e. a range of frequencies for which no propagating electromagnetic modes exist in any direction.
Preferably, the three dimensional pattern within the sample is formed by directing electromagnetic radiation from at least four coherent or partially coherent beams or sources at the sample of photosensitive material so as to intersect and interfere within the sample. Additionally, the sample may be irradiated more than once to generate a plurality of three dimensional patterns in the sample.
With the present invention the length scale of the periodicity within the sample of photosensitive material is dependent on the length scale of the periodicity of the interference pattern which in turn is dependent on the frequency of the incident radiation, on the refractive index of the photosensitive material and on the shape and direction of propagation of the interfering electromagnetic wavefronts within the sample. Three-dimensional periodicity with a submicron length scale can be produced in the sample without the need for expensive masks making the invention particularly suitable for the production of photonic crystal material for use in optical and electro-optical applications in the infra-red, visible optical or shorter wavelength regions of the electromagnetic spectrum.
Moreover, with the present invention a photonic crystal material is provided that has three dimensional periodicity of its refractive index for a thickness for at least 10 microns and more preferably at least 50 microns.